(1) Field of the Invention
This invention relates to a semiconductor device and a method for fabricating a semiconductor device and, more particularly, to a semiconductor device having multilayer wiring and a method for fabricating such a semiconductor device.
(2) Description of the Related Art
In a multilayer wiring structure which is widely used now in semiconductor devices and the like, signal propagation speed depends mainly on wire resistance and the parasitic capacitance of an insulating film between wires. If the distance between wires is wider than or equal to 1 μm, inter-wire capacitance is small and the influence on the operation speed of the entire device is slight. However, if the distance between wires is narrower than or equal to about 0.5 μm, then the influence on the operation speed of the entire device is considerable. In particular, it is expected that circuits in which the distance between wires is narrower than or equal to 0.2 μm will be fabricated in the future. In such cases, inter-wire capacitance will have a very great influence on the operation speed of devices. In addition, in recent years the integration levels of semiconductor devices have risen and the distance between wires has become narrower. Accordingly, conventional wire thickness leads to an increase in inter-wire capacitance. Inter-wire capacitance can be reduced by reducing wire thickness and narrowing an insulating film region between wires. However, if wire thickness is reduced, wire resistance increases and the operation speed of circuits cannot be increased. Therefore, the use of an insulating film having a low dielectric constant is now considered the most effective measure to reduce inter-wire capacitance, and attention is riveted on low-k materials the dielectric constants of which are about 2.0 to 2.5 (see, for example, Japanese Patent Laid-Open Publication No. 2005-136301).
Low-k materials include organic polyarylene, organic polyarylether, inorganic hydrogen silsesquioxane (HSQ), organic-inorganic hybrid methylsilsesquioxane (MSQ), and a mixture of HSQ and MSQ. A coating process is performed for forming a film of each of these materials. The low-k materials also include silicon oxycarbide. In this case, a chemical vapor deposition (CVD) process is performed by using an organosiloxane-based material as source gas. Furthermore, the method of lowering a dielectric constant by forming a cavity in an insulating film of a low-k material may be adopted in place of forming a film. In this case, a balance must be achieved between a drop in the dielectric constant and degradation in the mechanical strength of the film caused by forming the cavity and the application of this method to devices is under study.
In the multilayer wiring structure, usually minute wires are used in lower layers near elements where the above problem of inter-wire capacitance becomes more and more important. Accordingly, the use of a low-k material is strongly required especially in such regions. In addition, in the regions where such minute wires are used, inter-wire capacitance between two layers one of which is formed on the other cannot be neglected because of the spread of an electric field. Accordingly, there is now a great necessity to form insulating films of a low-k material not only between wires in the horizontal direction but also between wires in the vertical direction.
If insulating films of a low-k material are formed between wires and between layers in the multilayer wiring structure, a dielectric constant can be lowered. Compared with a conventional material such as a silicon oxide (SiO2) film, however, the mechanical strength is low. Insulating films in the multilayer wiring structure cannot be replaced excessively with insulating films of a low-k material. Accordingly, lowering the dielectric constants of a diffusion barrier film which prevents a wire material such as copper (Cu) from diffusing, a cap film formed on an insulating film, an etching stopper film formed beneath a layer to be etched, and the like which occupy narrower regions in the multilayer wiring structure has recently been required.
Silicon nitride (SiN), oxygen-doped silicon carbide (ODC) or the like which is formed by the CVD method can be used as the diffusion barrier film etc. The dielectric constants of SiN and ODC are about 7.0 and about 4.5 to 5.0, respectively.
Attention has recently been riveted on undoped silicon carbide (UDC) which is formed at low oxygen concentration or in an oxygen-free state by using organosilane as source gas and the carbon (C) content of which is higher as a material for a diffusion barrier film etc. The dielectric constant of UDC can be made 2.5 to 4.5. UDC used as a diffusion barrier film serves as a good barrier against copper diffusion. However, if UDC is used in the multilayer wiring structure, the following problem may arise.
To form the multilayer wiring structure by a damascene method, a wire groove is formed first in an insulating film. A thin barrier metal film of, for example, tantalum (Ta) is formed in the wire groove. Copper, being a conductive material, is then embedded in the wire groove to form a copper wire. The barrier metal film formed in the wire groove is conductive and prevents copper from diffusing from the copper wire to the insulating film. An insulating material is used for forming a diffusion barrier film, an etching stopper film, and the like on the copper wire and for forming a cap film on the insulating film before the formation of the wire groove. That is to say, these films are formed near the copper wire.
However, if UDC having a low dielectric constant is used for forming the diffusion barrier film and the like, copper contained in the copper wire may pierce through the barrier metal film in a region in a wiring structure where UDC is touching the barrier metal film, and diffuse into the insulating film, in the process of forming the multilayer wiring structure or at the time of the operation of the device. The diffusion of copper into the insulating film causes an increase in leakage current or wire resistance. Therefore, techniques for realizing a multilayer wiring structure in which a dielectric constant as a whole is low and in which the diffusion of a wire material into an insulating film is suppressed and a semiconductor device having such a multilayer wiring structure are greatly required.